Amyloid diseases (disorders of protein folding), or amyloidoses, are characterized by the accumulation of a peptide, including the Aβ peptide, existing as abnormal insoluble cross-β sheet fibrils or amyloid deposits in the affected organs. Amyloid diseases include, but are not limited to, Alzheimer's disease, Lewy body dementia, type 2 diabetes, Huntington's disease, Parkinson's disease, and chronic inflammation. Amyloidosis is also a common and serious complication of long-term hemodialysis for end-stage renal failure. Amyloidosis—in which amyloid deposits are the direct cause of death is responsible for about one per thousand of all deaths in developed countries.
Alzheimer's disease (AD) is the most common form of late-life dementia in adults, constituting the sixth leading cause of death in the United States (see, e.g., Alzheimer's Association, 2012 Alzheimer's Disease Facts and Figures, Alzheimer's & Dementia, Volume 8, Issue 2, available at www.alz.org/downloads/facts_figures_2012.pdf). AD is also globally the most common cause of dementia; the increasing numbers of AD patients worldwide is one of the most pressing medical issues of the 21st century (see, e.g., Alzheimer's Disease International World Alzheimer Report 2011, The benefits of early diagnosis and intervention, Martin Prince, Renata Bryce and Cleusa Ferri, Institute of Psychiatry, King's College London, UK, published by Alzheimer's Disease International (ADI), September 2011, available at www.alz.co.uk/research/world-report-2011). Approximately 5% of the population over 65 years old is affected by this progressive degenerative disorder that is characterized by memory loss, confusion and a variety of cognitive disabilities.
Neuropathologically, AD is characterized by four major lesions: a) intraneuronal, cytoplasmic deposits of neurofibrillary tangles (NFT), b) parenchymal amyloid deposits called neuritic plaques, c) cerebrovascular amyloidosis, and d) synaptic and neuronal loss. One of the key events in AD is the deposition of amyloid as insoluble fibrous masses (amyloidogenesis) resulting in extracellular neuritic plaques and deposits around the walls of cerebral blood vessels. The major constituent of the neuritic plaques and congophilic angiopathy is Aβ, although these deposits also contain other proteins such as glycosaminoglycans and apolipoproteins. The major constituent of NFTs is the tau protein in an abnormal conformation which is hyperphosphorylated. NFTs are also considered amyloid deposits, wherein tau (pTau) is present in toxic oligomeric forms that might or might not be soluble. Hence, in AD two different biochemical types of proteins are deposited and at least two major protein toxic oligomeric forms are involved.
Aβ is a 4.14.3 kD hydrophobic peptide that is encoded on chromosome 21 as part of a much longer amyloid precursor protein APP (Muller-Hill et al., Nucleic Acids Res 1989; 17:517-522). The APP protein starts with a leader sequence (signal peptide), followed by a cysteine-rich region, an acidic-rich domain, a protease inhibitor motif, a putative N-glycosylated region, a transmembrane domain, and finally a small cytoplasmic region. The Aβ sequence begins close to the membrane on the extracellular side and ends within the membrane. Two-thirds of Aβ faces the extracellular space, and the other third is embedded in the membrane (Kang et al., Nature 1984; 325:733-736, Dyrks et al., EMBO J 1988; 7:949-957). Several lines of evidence suggest that amyloid may play a central role in the early pathogenesis of AD (Soto et al., 1994; 63:1191-1198).
Evidence that amyloid may play an important role in the early pathogenesis of AD comes primarily from studies of individuals affected by the familial form of AD (FAD) or by Down's syndrome. Down's syndrome patients have three copies of the APP gene and develop AD neuropathology at an early age (Wisniewski et al., Ann Neurol 1985; 17:278-282). Genetic analysis of families with hereditary AD revealed mutations in chromosome 21, near or within the Aβ sequence (Ghiso et al., Adv. Drug Deliv. Rev. 2002; 54(12):1539-51), in addition to mutations within the presenilin 1 and 2 genes. Moreover, it was reported that transgenic mice expressing high levels of human mutant APP progressively develop amyloidosis in their brains (Games et al., Nature 1995; 373:523-527). These findings appear to implicate amyloidogenesis in the pathophysiology of AD. In addition, Aβ fibrils are toxic to neurons in culture, and to some extent when injected into animal brains (Sigurdsson et al., Neurobiol Aging 1996; 17:893-901; Sigurdsson et at, J Neuropathol Exp Neurol. 1997; 56:714-725).
Furthermore, several other pieces of evidence suggest that the deposition of Aβ is a central triggering event in the pathogenesis of AD, which subsequently might be linked or related to NFT formation and neuronal loss. The amyloid deposits in AD share a number of properties with all the other cerebral amyloidoses, such as the prion related amyloidoses, as well as the systemic amyloidoses. These characteristics are: 1) being relatively insoluble; 2) having a high degree of β-sheet secondary structure, which is associated with a tendency to aggregate or polymerize; 3) ultrastructurally, the deposits are mainly fibrillary; 4) the presence of certain amyloid-associating proteins such as amyloid P component, proteoglycans and apolipoproteins; and 5) deposits show a characteristic apple-green birefringence when viewed under polarized light after Congo red staining.
The same peptide that forms amyloid deposits in the AD brain was also found in a soluble form (sAβ) normally circulating in human body fluids (Seubert et al., Nature 1992; 359:325-327; Shoji et al., Science 1992; 258:126-129). sAβ was reported to pass freely from the brain to the blood (Ji et al., Journal of Alzheimer's Disease 2001; 3:23-30; Shibata et al., J Clin Invest 2000; 106: 1489-99; Ghersi-Egea et al., J Neurochem 1996; 67(2):880-3; Zlokovic et al., Biochem Biophys Res Commun 1994; 205:1431-1437), reported that the blood-brain barrier (BBB) has the capability to control cerebrovascular sequestration and transport of circulating sAβ, and that the transport of sAβ across the BBB was significantly increased in guinea pigs when sAβ was perfused as a complex with apolipoprotein J (apoJ). The sAβ-apoJ complex was found in normal cerebrospinal fluid (CSF; Ghiso et al., Biochem J 1993; 293:27-30; Ghiso et al., Mol Neurobiol 1994; 8:49-64) and in vivo studies indicated that sAβ is transported with apoJ as a component of the high density lipoproteins (HDL) in normal human plasma (Koudinov et al., Biochem Biophys Res Commun 1994; 205:1164-1171). It was also reported by (Zlokovic et al., Proc Natl Acad Sci USA 1996; 93:4229-4234), that the transport of sAβ from the circulation into the brain was almost abolished when the apoJ receptor, gp330, was blocked. It has been suggested that the amyloid formation is a nucleation-dependent phenomena in which the initial insoluble “seed” allows the selective deposition of amyloid (Jarrett et al., Cell 1993; 73:1055-1058; Jarrett et al., Biochemistry 1993; 32:4693-4697).
Therapeutic strategies proposed for treating AD and other amyloid diseases include the use of compounds that affect processing of the amyloid-β precursor protein (Dovey et al., J Neurochem. 2001; 76:173-182), or that interfere with fibril formation or promote fibril disassembly (Soto et al., Nat Med 1998; 4:822-826; Sigurdsson et al., J Neuropath Exp Neurol 2000; 59:11-17; Findeis M A., Biochim Biophys Acta 2000; 1502:76-84), as well as the administration of Aβ antibodies to disassemble fibrillar Aβ, maintain Aβ solubility and to block the toxic effects of Aβ (Frenkel et al, J Neuroimmunol 1999; 95:136-142). However, recently a Phase II clinical trial using a vaccination approach where Aβ1-42 was injected into individuals in the early stages of AD was terminated because of cerebral inflammation observed in some patients.
Taken together, despite some advances in the art, to date, there is no cure or effective therapy for reducing a patient's amyloid burden or preventing amyloid deposition in AD and other amyloid diseases. Thus, there exists a need in the art for developing effective methods for reducing a patient's amyloid burden.